Stainless steel resistant to stress-corrosion cracking



United States Patent 3,512,960 STAINLESS STEEL RESISTANT T0 STRESS- CORROSION CRACKING Richard R. Brady, Butler, and Kenneth G. Brickner,

Wilkinsburg, Pa., assignors to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Jan 28, 1963, Ser. No. 254,493 Int. Cl. C22c 39/20 US. Cl. 75-128 4 Claims This invention relates to a high strength martensitic precipitation-hardenable stainless steel with resistance to stress-corrosion cracking.

In the aircraft, missile, chemical, and other industries a need exists for a hardenable stainless steel with relatively high strength at room and elevated temperatures and with good resistance to stress-corrosion cracking upon exposure to chloride-type environments such as a marine atmosphere. The high-strength hardenable stainless steels now used for many engineering applications in the abovementioned industries do not have satisfactory resistance to stress-corrision cracking in chloride-type environments when these steels are heat treated to high-strength levels and used at relatively high-stress levels. The results of many stress-corrosion tests have indicated that, generally, the higher the yield strength of a steel, the more susceptible the steel is to stress-corrosion cracking, and generally, the higher the stress applied to the steel, the more susceptible the steel is to stress-corrosion cracking.

Accordingly, it is a principal object of this invention to provide a stainless steel which develops relatively high strength after suitable heat treatment and has good resistance to stress-corrosion cracking in chloride-type environments. The mode of accomplishment of this and other objects will become apparent from the ensuing description of examples and test results.

Typically, the steel of this invention is a relatively low carbon, nominally percent chromium and 6 percent nickel, martensitic stainless steel, in which resistance to stress-corrosion cracking is imparted by the addition of titanium and molybdenum, and the restriction of aluminum and nitrogen contents.

The steel contains sufficient chromium (about 15%) for good resistance to atmospheric corrosion and elevatedtemperature oxidation. In addition, the steel contains the austenitizing elements, carbon and nickel, in such proportions that these elements balance the ferritizing elements, chromium molybdenum, and titanium, and enable the austenite to martensite transformation to occur upon cooling. Furthermore, the titanium and carbon are controlled so that an aging as high as 1100 F., a critical dispersion of titanium carbide and other titanium containing intermetallic compounds are precipitated to develop the relatively high yield and tensile strengths needed for many engineering applications. The titanium, in addition to its function as a precipitation-hardening element, combines with the nitrogen in the steel and thereby reduces the amount of nitrogen in solid solution. We believe the amount of nitrogen in solid solution is one of the factors that determine the degree to which a martensitic matrix will be susceptible to stress-corrosion cracking. Thus, a low-nitrogen martensitic structure is generally less susceptible to stress-corrosion cracking in a chloride-type environment than is a high-nitrogen martensitic structure. The steel also contains molybdenum for strength at high temperatures and improved resistance to stress-corrosion cracking. Molybdenum probably gives improved resistance to stress-corrosion cracking because it increases the resistance of the steel to pitting corrosion thus retarding the formation of pits which act as stress raisers. Cobalt may also be added to the steel, preferably in the range of about 2 to 6%, Without any deleterious ice effect on the stress-corrosion resistance of the steel in a chloride-type environment. Cobalt, like molybdenum, improves elevated-temperature strength, and because cobalt is an austenitizing element, it helps balance the ferritizing elements and also reduces the amount of delta ferrite in the new steel, thereby improving transverse ductility and hot workability.

In addition to the above elements which compose the new steel, it is extremely critical that the aluminum and nitrogen contents be kept at extremely low levels. Without these restrictions with respect to aluminum and nitrogen the steel will not be resistant to stress-corrosion cracking in chloride-type environments, as evidenced by the examples of conventional steels and their stress-corrosion properties shown hereafter.

Specific operative ranges of various steel-making elements, in their broad and preferred embodiments within the principles of the invention, are as follows:

Broad (percent) Preferred (percent) 0. 03-0. 15 0 06-0. 12 up to 1.0 0 50-0. 0. 04 max. 0 04 max 0. 04 max. 0 04 max Up to 1.0 0 40-0. 60 5. 0-7. 0 5. 5-6. 5 14. 0-16. 5 14. 5-16. 0 1. 0-3.0 2 25-2. 75 0 60-1. 25 0. -1. 15 0-6. 0 2. 0-6. 0 0. 08 max. 0 05 max Nitrogen- 0 015 max. 0 015 max with the balance predominately iron, and residual amounts of other elements.

To develop relatively high strength, the aforementioned balance of austenitizing and ferritizing elements must be maintained. That is, if the ferritizing elements are on the high side of the composition range, the austenitizing elements must also be on the high side of the range.

The preferred heat treatment for the new steel is as follows: (1) anneal for 5 to 30 minutes at 1750" to 2050 F., air cool to room temperature, (2) if necessary to effect the austenite to martensite transformation, cool to about F. for a minimum of 2 hours, and (3) age at 850 to 1100 F. for a minimum of 30 minutes.

Examples of three steels, within the composition of the invention, together with their tensile properties and stresscorrosion test results, are presented in Tables I and I-A.

TABLE I.-OOMPOSITION OF THE NEW STEEL (PERCENTI Example No 1 2 3 Carbon 0. 11 0. 10 0. 11 Manganese. 0. 70 0. 60 0. 63 Phosphorus 0. 010 O. 011 0. 010 Sulfur 0.030 0 034 0. 035 0.60 0.54 0.57

Nitrogen 0. 013 0. 013 0. 010

with the balance predominately iron, and residual amounts of other elements.

TABLE 1-A.TENSI'LE PROPERTIES AND CORROSION TEST RESULTS OF TABLE I COMPOSITIONS Specimens had not failed at expiration of time indicated.

Notes (1) The stress-corrosion tests were conducted in a marine atmosphere (Kure Beach, North Carolina) upon multiple specimens of 0.05 inch thickness, exposed while under a stress equal to 75 percent of their yield strength.

(2) The steels reported in Table I-A had been heat treated as follows:

Steel 1.Annealed at 1750 F. for 15 minutes, air cooled to room temperature, and then aged at 1000 F. for 30 minutes Steel 2.Annealed at 1850 F. for 15 minutes, air cooled to room temperature, and then aged at 1000 F. for 30 minutes Steel 3.-Annealed at 1950 F. for 15 minutes, air cooled to room temperature, refrigerated at 100 F. for 6 hours, and then aged at 1000 F. for 30 minutes For comparative purposes, stress-corrosion test results are presented in Table II-A, below of some steels whose compositions are outlined in Table II, which steels are generally similar in composition to the steel of the invention but fall somewhat outside of the composition limits of the principles thereof.

TABLE II.COMPOSITIONS OF REPRESENTATIVE COMPARATIVE STEELS (PERCENT) B C D E Nitrogen 0. 03 0. 03 0. 1 0. 02 0. 003

with the balance predominately iron, and residual amounts of other elements.

TABLE IIA.-TENSILE PROPERTIES AND CORROSION TEST RESULTS OF TABLE II COMPOSITIONS Room-temperature tensile properties Note-The stress-corrosion tests were conducted in a marine atmosphere (Kure Beach, North Carolina) upon multiple specimens of 0.05 inch thickness, exposed while under a stress equal to 75 percent of their yield strength.

Steels A, B, C, and D are commercially available steels, while Steel E represents a low nitrogen laboratory melt of a steel otherwise analogous to Steel D. A comparison of the test results of Tables I-A and II-A indicates the relative stress-corrosion resisting advantages of the steel of the invention.

While we have shown and described several specific embodiments of our invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims.

We claim:

1. A martensitic precipitation-hardenable stainless steel with resistance to stress-corrosion cracking, comprising Percent Carbon 0.06-0.12 Manganese 0.50-0.80 Silicon 0.40-0.60 Nickel 5.5-6.5 Chromium 14.5-16.0 Molybdenum 2.25-2.75 Titanium 0.85-1.15 Aluminum (maximum) 0.05 Nitrogen (maximum) 0.015

with the balance iron and residual amounts of other elements which do not adversely affect the properties.

2. A martensitic precipitation-hardenable stainless steel with resistance to stress-corrosion cracking, comprising with the balance iron and residual amounts of other elements which do not adversely alfect the properties.

3. A martensitic, precipitation hardenable stainless steel consisting essentially of .03% to .05 carbon, traces to .3% manganese, traces to .2% silicon, 14.5% to 15.5% chromium, 5.25% to 6.5% nickel, .6% to 1% titanium, 1% to 2% molybdenum and the balance essentially iron.

4. A martensitic precipitation hardenable stainless steel consisting essentially of 0.03 to 0.12% carbon, traces to 0.6% manganese, traces to 0.35% silicon, 14.0 to 16.5% chromium, 5.0 to 7.0% nickel, 0.6 to 1.25% titanium and 1.0 to 3.0% molybdenum.

References Cited UNITED STATES PATENTS 2,879,194 3/ 1959 Eichelberger -128 2,597,173 5/1952 Patterson 75--128 2,083,524 6/1937 Payson 75-128 3,251,683 5/1966 Hammond 14837 HYLAND BIZOT, Primary Examiner US. Cl. X.R. l48-37 

1. A MARTENSITIC PRECIPITATION-HARDENABLE STAINLESS STEEL WITH RESISTANCE TO STRESS-CORROSION CRACKING, COMPRISING 